High-Efficiency Semitransparent Organic Photovoltaics Containing Vertical Multiheterojunctions

Although semitransparent organic photovoltaics (ST-OPVs) are attractive components of various building-integrated photovoltaic applications, there is an intrinsic trade-off in their power conversion efficiencies (PCEs) and average visible transmissions (AVTs)─because the photocurrent generated usually requires substantial absorption in the visible light that determines transmission. In this paper, we describe a vertically stacked tunable multiheterojunction strategy toward highly efficient ST-OPVs that simultaneously maintain high AVTs. The vertical triheterojunctions of the active layer comprise one polymer donor, D18, and two small molecule acceptors, Y1 and Y6, that were formed with a sequentially deposited (SD) method. The triheterojunction structures not only introduced cascading energy levels of their lowest unoccupied molecular energy levels that aligned in parallel to the charge extraction direction but also can be tuned by varying their relative thicknesses, thereby significantly improving charge transport in the ST-OPVs without severely suffering AVT losses. We demonstrate the universality and broad tunability of this technique for balancing the PCEs and AVTs of the devices. Our ST-OPVs achieved champion PCEs as high as 13.9% at an AVT of 22.8% (100 nm active layer with deposited trilayers having D18, Y1, and Y6 at 45, 5, and 50 nm, respectively) and 13.5% at an AVT of 23.8% (100 nm active layer with deposited trilayers having D18, Y1, and Y6 at 40, 10, and 50 nm, respectively), while the PCE of the device with a bulk heterojunction (BHJ) D18/Y6 (1:1 wt) active layer at 100 nm is 12.3% at an AVT of 17.0%. Relative to the conventional BHJ devices, the PCEs of the both types of SD trilayer devices increased by at least 10%, while the AVTs increased by at least 25%, indicating the effectiveness of the vertical multiheterojunction structures. These efficiencies of the SD trilayer devices are among one of the best records for such ST-OPVs at comparable AVTs.